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Date: Mon, 29 Jun 2020 15:44:39 +0200
From: Cornelia Huck <cohuck@redhat.com>
Subject: Re: [PATCH v3 1/1] s390: virtio: let arch accept devices without
 IOMMU feature
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To: Pierre Morel <pmorel@linux.ibm.com>
Cc: Halil Pasic <pasic@linux.ibm.com>, linux-kernel@vger.kernel.org, borntraeger@de.ibm.com, frankja@linux.ibm.com, mst@redhat.com, jasowang@redhat.com, kvm@vger.kernel.org, linux-s390@vger.kernel.org, virtualization@lists.linux-foundation.org, thomas.lendacky@amd.com, david@gibson.dropbear.id.au, linuxram@us.ibm.com, heiko.carstens@de.ibm.com, gor@linux.ibm.com

On Mon, 29 Jun 2020 15:14:04 +0200
Pierre Morel <pmorel@linux.ibm.com> wrote:

> On 2020-06-19 11:20, Cornelia Huck wrote:
> > On Thu, 18 Jun 2020 00:29:56 +0200
> > Halil Pasic <pasic@linux.ibm.com> wrote:
> >   
> >> On Wed, 17 Jun 2020 12:43:57 +0200
> >> Pierre Morel <pmorel@linux.ibm.com> wrote:  

> >>> @@ -179,6 +194,13 @@ int virtio_finalize_features(struct virtio_device *dev)
> >>>   	if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
> >>>   		return 0;
> >>>   
> >>> +	if (arch_needs_virtio_iommu_platform(dev) &&
> >>> +		!virtio_has_feature(dev, VIRTIO_F_IOMMU_PLATFORM)) {
> >>> +		dev_warn(&dev->dev,
> >>> +			 "virtio: device must provide VIRTIO_F_IOMMU_PLATFORM\n");  

[Side note: wasn't there a patch renaming this bit on the list? I think
this name is only kept for userspace compat.]

> >>
> >> I'm not sure, divulging the current Linux name of this feature bit is a
> >> good idea, but if everybody else is fine with this, I don't care that  
> > 
> > Not sure if that feature name will ever change, as it is exported in
> > headers. At most, we might want to add the new ACCESS_PLATFORM define
> > and keep the old one, but that would still mean some churn.
> >   
> >> much. An alternative would be:
> >> "virtio: device falsely claims to have full access to the memory,
> >> aborting the device"  
> > 
> > "virtio: device does not work with limited memory access" ?
> > 
> > But no issue with keeping the current message.
> >   
> 
> If it is OK, I would like to specify that the arch is responsible to 
> accept or not the device.
> The reason why the device is not accepted without IOMMU_PLATFORM is arch 
> specific.

Hm, I'd think the reason is always the same (the device cannot access
the memory directly), just the way to figure out whether that is the
case or not is arch-specific, as with so many other things. No real
need to go into detail here, I think.